There are no mammals, birds, reptiles or amphibians that naturally have only one eye.
Flatfish like the flounder, commonly thought of as having only one eye, in fact have two.
One migrates to join the other on the right or left side.
However, there are several orders of one-eyed animals among the invertebrates. Among them are some “water fleas,” which are actually crustaceans.
Another one-eyed crustacean, appropriately named the cyclops, is a common inhabitant of pond scum in stagnant bodies of water.
It has compound eyes like those of the fly, but only one per customer.